1. Field of the Invention
This invention relates to a process for producing a foamed article from pre-foamed particles of a polyolefin resin.
2. Description of the Prior Art
Some methods have been proposed in the past which comprise crosslinking and pre-foaming a polyolefin resin as a base resin to produce pre-foamed crosslinked polyolefin particles, filling the pre-foamed particles in a mold, and expanding them by heating, thereby to obtain a molded article having the same configuration as the mold. For example, Japanese Patent Publication No. 22951/1976 discloses a method which comprises keeping pre-foamed particles of a polyolefin resin for a predetermined period of time in an inorganic gas, such as air, maintained at a high pressure to include the inorganic gas at a pressure of at least 1.18 atmospheres within the particles, filling the particles in a mold, and expanding them under heat. This method, however, has the disadvantage that when a foamed article having a large thickness is produced by using this known method, adhesion among particles in the inside of the foamed article becomes poor and the quality of the product is not satisfactory. Furthermore, this known method can be applied only to those polyolefin resins which have a low softening temperature, for example low-density polyethylene, and this leads to the defect that when this method is applied to a resin having a high softening temperature such as polypropylene, the resulting product has poor adhesion not only at its core portion but also at its surface layer portion.